The semiconductor industry has seen tremendous advances in technology in recent years which have permitted dramatic increases in circuit density and complexity, and equally dramatic decreases in power consumption and package sizes. Present semiconductor technology now permits single-chip microprocessors with many millions of transistors, operating at speeds of tens (or even hundreds) of MIPS (millions of instructions per second) to be packaged in relatively small, air-cooled semiconductor device packages. A by-product of such high-density and high functionality in semiconductor devices has been the demand for increased numbers of external electrical connections to be present on the exterior of the semiconductor packages, for connecting the packaged device to external systems (such as via a printed circuit board). Traditional packaging techniques involving pins, leads and the like are inadequate to the task.
A number of newer interconnection technologies have been used, with varying degrees of success, to increase connection densities above those of older techniques (e.g., the relatively low-density DIP or Dual Inline Package connection technique). These newer techniques are represented primarily by solder-reflow surface-mount technologies such as: PQFP (Plastic Quad Flat Pack), SOIC (Small Outline Integrated Circuit), LCC's (Leadless Chip Carriers), J-lead packages, etc., although a few high-density non-surface-mount techniques (e.g., PGA--Pin Grid Array) have also been used. Among the newer surface mounting techniques is a type of package on which a plurality of ball-shaped conductive bump contacts (usually solder, or other similar conductive material) are disposed. Such packages are occasionally referred to as "Ball Grid Array" (BGA) or "Area Grid Array" packages--the solder balls on the exterior surface of the package typically being arranged in a rectangular array.
A typical BGA package is characterized by a large number (potentially several hundreds) of solder balls disposed in an array on a surface of the package. The BGA package is assembled to a matching array of conductive pads (presumably connected to other circuitry) on a substrate or circuit board. Heat is applied to reflow the solder balls (bumps) on the package, thereby wetting the pads on the substrates and, once cooled, forming electrical connections between the package (and the semiconductor device contained in the package) and the substrate.
The introduction of the Ball Grid Array (BGA) package to the semiconductor industry has brought with it several new package manufacturing and assembly problems. One of the more significant problems is finding an efficient, cost-effective technique for applying the solder ball contacts to the package surface. The package surface is usually formed from an electrically insulating material (e.g., printed circuit board material) with a pattern of metallized pads disposed thereupon connecting to circuity on a semiconductor device within the package. Several methods are currently used to dispose solder balls or conductive bump contacts to these package pads.
One method of disposing solder balls or conductive bump contacts on package pads involves the application of solder flux to the package pads, then placing pre-formed solder balls onto the package pads, either individually or en masse, with the aid of a fixture or a "pick-and-place" apparatus similar to those used for circuit board assembly. The package is then heated to the melting point of the solder ball alloy which will then wet the metallic surface of the pads and make electrical contact therewith.
This pick-and-place method requires the precision handling of massive quantities of solder balls. As the connection counts of packages increase, hundreds or even thousands of balls must be manipulated in this fashion for each package.
An alternative method of disposing solder balls or conductive bump contacts on package pads involves using a printing or dispensing fixture to apply measured quantities of solder paste (a mixture of fine solder particles in a flux-containing medium) to the package contact pads. Upon exposure to heat, the solder melts and surface tension causes the solder to assume a generally spherical shape. Once cooled, the spherical shapes form the "ball grid" contacts of the package. Evidently, ball bump type contacts formed in this manner, being generally spherical, will exhibit a 1:1 aspect ratio of height-to-width. Even if hemispherical, the ball bump type contacts will have a height:width ratio on the order of 0.5:1. In certain applications, it would be desirable that the external package contacts have a height:width ratio in excess of 1:1 (e.g., 2:1).
Another technique for disposing solder balls or conductive bump contacts on package pads involves using printed solder paste, then placing a preformed ball, which is essentially a combination of the two techniques described hereinabove. In this technique, solder is printed onto the contact pad to form an "adhesive" on the contact pad, then a pre-formed solder ball is placed onto the contact pad.
Difficulties with the technique of measuring or dispensing quantities of solder paste on pads to form ball bump contacts include dealing with the rheological characteristics (elasticity, viscosity, plasticity) of the solder paste, accurately controlling the volume of solder paste after dispensing and reflow, and the shape of the final ball contact. The shape of the ball contact can be affected by such factors as surface tension of the molten solder and the amount of wettable exposed metal area of the contact pad.
The generally spherical shape assumed by solder balls formed as described hereinabove prevents the formation of "tall" contacts by ordinary means. In certain applications, tall solder bumps or ball contacts can be used to great advantage in reflow assembly (e.g., of a packaged semiconductor device to a printed circuit board). As mentioned above, in general it is difficult to form contacts with height-to-width ratios (aspect ratios) of greater than 1:1. Some techniques involving "building up" of solder contact height in a series of process steps have managed to produce tall contacts, but such techniques are expensive and cumbersome in high-volume production.
Consistency in the height of solder ball contacts is another critical factor for successful assembly of BGA-type packages to circuit boards. If one or more balls are significantly shorter than others (usually due to an insufficient amount of solder paste deposited on one or more conductive pads prior to contact formation) it becomes highly likely that these smaller (shorter) contacts will completely miss their mating contact pads (on the circuit board) and will fail to form an electrical connection between the packaged semiconductor device and the underlying substrate (e.g., printed circuit board). Hence, quality control for BGA packages is critical, since proper electrical connections between the BGA package and the substrate to which it is assembled are formed only if each and every one of the solder ball contacts reflows correctly and wets its associated conductive pad on the substrate. Defective assemblies of packages to substrates can be difficult or impossible to repair after assembly if connections are not properly formed. Even prior to assembly, the correction of improperly formed solder ball type contacts on the exterior of a package can be very difficult and involves, initially, careful quality control inspection of the ball bump contacts prior to assembly of the packaged device to a substrate.
As the volume of packages produced by the aforementioned methods increases, the complexity of the manufacturing processes becomes an obstacle to high manufacturing rates. In order to avoid high scrap rates, high machine accuracy must be maintained, raw material properties (e.g., solder paste and pad metal) must be carefully controlled, and numerous process parameters (e.g., amount of solder paste dispensed, size of conductive pads, temperature, shape and size of ball contact) must be monitored.
Further complicating matters, in order to accommodate different package configurations (e.g., different size packages, different array spacing of the ball bump contacts, etc.), it may be necessary to change numerous parts of the manufacturing equipment (tooling). Generally speaking, complicated setup and tooling changes tend to increase downtime, thereby increasing production cost.